Blender 2.8: Target UV map is half the size of the source UV map
Solved this myself by some experimentation. What worked was to isolate the parts in a previous save before joining parts and check each mesh for Cleanup issues. I first checked for loose geometry and then to merge vertices or as they were called in v2.79 duplicated vertices. I did find issues. Not a lot but a few more than I expected to. I also checked for scaling and rotation issues but didn't find any.The last thing I did was to add 1 pixel's worth for margins. I baked and all went as expected with the target bake matching the source UV map whereas before the target UV map had its own mind and resembled little of the source. I'm including the redo bake of the house_parts uv map just to show it worked.

So in conclusion what I believe was at issue was the mesh irregularities. I'm concluding as a newbie (you can see my site to determine my skill level at All Aboard) that if baking is a goal that the mesh parts be free from any defects. Seems a rather obvious conclusion but in my situation there were a number of parts and I thought they all had been checked. The problem I had was nothing in the problem I listed above suggested there could be mesh issues. So lesson learned.

Here's the bake redo.